U.S. patent number 7,039,119 [Application Number 10/267,602] was granted by the patent office on 2006-05-02 for method and apparatus for combining transponders on multiple satellites into virtual channels.
This patent grant is currently assigned to Virtual Satellite Corporation. Invention is credited to Robert F. Friedman.
United States Patent |
7,039,119 |
Friedman |
May 2, 2006 |
Method and apparatus for combining transponders on multiple
satellites into virtual channels
Abstract
A satellite communications system provides an information
channel between remotely located transmitters and receivers. A
virtual satellite system provides the same service, but divides the
signal either in power or in data content into subchannels such
that any particular signal is conducted to the intended receiver
via a plurality of traditional satellite channels. The receiving
terminal accepts the plurality of signals simultaneously from a
possible plurality of satellites, combining the subchannels
comprising the virtual channel into the original signal content as
if conducted via a single channel. The receiving antenna system
receives satellite subchannel signals from a plurality of
directions using multiple antennas or a single antenna with
multi-direction capability. Prior to signal combining, the receiver
necessarily time-synchronizes the plurality of subchannels by
introducing time delay in some channels before combining the
subsignals into the original composite. A timing signal present in
the virtual satellite system assists the receiver in determining
the amount of delay to apply to each incoming signal. The timing
signal is either a separate carrier or an additional modulation on
the existing information-bearing carrier.
Inventors: |
Friedman; Robert F.
(Fayetteville, AR) |
Assignee: |
Virtual Satellite Corporation
(Fayetteville, AR)
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Family
ID: |
26754696 |
Appl.
No.: |
10/267,602 |
Filed: |
October 8, 2002 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20030031265 A1 |
Feb 13, 2003 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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09723877 |
Nov 28, 2000 |
6549582 |
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09243910 |
Feb 3, 1999 |
6154501 |
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60073619 |
Feb 4, 1998 |
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60113693 |
Dec 24, 1998 |
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Current U.S.
Class: |
375/260; 375/295;
375/316 |
Current CPC
Class: |
H04B
7/18513 (20130101); H04B 7/18515 (20130101) |
Current International
Class: |
H04L
27/28 (20060101); H04L 27/12 (20060101); H04L
27/14 (20060101) |
Field of
Search: |
;375/260,211,267,295,316,299,347
;455/13.1-13.3,12,61,59,101,132,137,139 ;370/480,315,316,326 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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99908092-2 |
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Feb 1999 |
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EP |
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WO 99/40693 |
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Aug 1999 |
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WO |
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Primary Examiner: Bocure; Tesfaldet
Attorney, Agent or Firm: DLA Piper Rudnick Gray Cary US
LLP
Parent Case Text
RELATED APPLICATIONS
This application is a continuation application of U.S. patent
application Ser. No. 09/723,877 now U.S. Pat. No. 6,549,582, which
is in turn a continuation of U.S. Pat. No. 6,154,501 filed on Feb.
3, 1999. U.S. Pat. No. 6,154,501 claims priority from U.S.
Provisional Application No. 60/073,619 filed on Feb. 4, 1998 and
U.S. Provisional Application No. 60/113,693 filed on Dec. 24, 1998.
Claims
I claim:
1. A method for transmitting data to a receiving station over at
least one satellite transponder, the method comprising: dividing an
original signal having a first bandwidth into a plurality of
subchannels, each subchannel having a bandwidth smaller than the
first bandwidth and including a unique information; adding a
synchronization signal to each subchannel so that the plurality of
subchannels are synchronized and combined to recreate the original
signal after being transmitted through the satellite
transponder;and adding a propagation delay indicator to each of the
subchannels, the propagation delay indicator allowing the unique
information in the plurality of subchannels to be synchronized
before being combined into a reconstructed version of the original
signal.
2. The method of claim 1 further comprising adding the
synchronization signal so that the synchronization signal is within
the bandwidth of each subchannel and no additional bandwidth
allocation is required.
3. The method of claim 1 further comprising adding the
synchronization signal on a separate frequency allocation.
4. The method of claim 1 further comprising associating each
subchannel with a unique amount of propagation delay.
5. The method of claim 1 wherein the original signal and the
subchannels are analog signals.
6. A method for transmitting data to a receiving station over at
least one satellite transponder, the method comprising: dividing an
original signal having a first bit rate into a plurality of
subchannels, each subchannel carrying information at the same bit
rate as the first bit rate and including a unique information;
adding a synchronization signal to each subchannel so that the
plurality of subchannels are synchronized and combined to recreate
the original signal after being transmitted through the satellite
transponder; and adding a propagation delay indicator to each
subchannel, the propagation delay indicator allowing the unique
information in the plurality of subchannels to be synchronized
before being combined into a reconstructed version of the original
signal.
7. The method of claim 6 wherein the synchronization signal is
added to the plurality of subchannels within the bandwidth of each
subchannel so that no additional bandwidth allocation is
required.
8. The method of claim 6 further comprising associating each
subchannel with a unique amount of propagation delay.
9. A plurality of subchannels for communication, each of the
plurality of subchannels comprising: a subchannel bandwidth smaller
than a bandwidth of an original signal that was divided into the
plurality of subchannels; a unique information within the
subchannel bandwidth, wherein combination of the unique information
in each of the subchannels is substantially equal to an information
in the original signal; a common timing signal within the
subchannel bandwidth, the common timing signal allowing the
plurality of subchannels to be properly combined into a
reconstructed version of the original signal after the subchannels
are transmitted via at least one satellite transponder; and a
propagation delay indicator within the bandwidth of each
subchannel, the propagation delay indicator allowing the unique
information in the plurality of subchannels to be synchronized
before being combined into the reconstructed version of the
original signal.
10. The plurality of subchannels of claim 9, wherein the unique
information and the synchronization signal are carried within a
single frequency range allocation.
11. A plurality of electrical subsignals for communication, wherein
an electrical signal having a first bandwidth is divided into the
plurality of electrical subsignals for transmission over a
plurality of communication channels, wherein the plurality of
electrical subsignals are embodied in a carrier wave, and each of
the plurality of electrical subsignals has a bandwidth smaller than
the first bandwidth, the plurality of electrical subsignals further
comprising some information unique to that subsignal so that the
plurality of electrical subsignals are identifiable and
distinguishable, wherein a synchronization signal is added to the
plurality of electrical subsignals within the bandwidth of each
subsignal so that no additional bandwidth allocation is required to
transmit the synchronization signal, the plurality of tuners
receiving the synchronization signal to measure relative
propagation delays of the subsignals, the amount of delay in the
plurality of delay means is set, and the plurality of tuners are
redirected to receive the associated information-bearing
subsignals; whereby the plurality of electrical subsignals are
received, synchronized, and combined for re-creating the electrical
signal.
12. The claim of 11, wherein the plurality of tuners receive the
synchronization signal without employing a coded chip.
13. A plurality of electrical subsignals for communication, wherein
an electrical signal having a first bit rate is divided into the
plurality of electrical subsignals for transmission over a
plurality of communication channels, wherein the plurality of
electrical subsignals are embodied in a carrier wave, and each of
the plurality of electrical subsignals carries identical
information at the same bit rate as the first bit rate, the
plurality of electrical subsignals comprises some information
unique to that subsignal so that the plurality of electrical
subsignals are identifiable and distinguishable wherein a
synchronization signal is added to the plurality of electrical
subsignals within the bandwidth of each subsignal so that no
additional bandwidth allocation is required to transmit the
synchronization signal and wherein a plurality of tuners receive
the synchronization signal to measure relative propagation delays
of the subsignals, the amount of delay in a plurality of delay
means is set, and the plurality of tuners are redirected to receive
the associated information-bearing subchannels; whereby the
plurality of electrical subsignals are received and combined for
re-creating the electrical signal.
14. The claim of 13, wherein the synchronization signal is added to
the plurality of electrical subsignals without employing a coded
chip.
15. A plurality of analog subsignals for communication, wherein an
analog signal having a first power is divided into the plurality of
analog subsignals for transmission over a plurality of
communication channels, wherein the plurality of analog subsignals
are embodied in a carrier wave, and each of the plurality of analog
subsignals has a power smaller than the first power, wherein a
synchronization signal is added to the plurality of analog
subsignals within the bandwidth of each subsignal so that no
additional bandwidth allocation is required to transmit the
synchronization signal, wherein a plurality of tuners receive the
synchronization signal to measure relative propagation delays of
the subsignals, the amount of delay in a plurality of delay means
is set, and the plurality of tuners are redirected to receive the
associated information-bearing subsignals; and whereby the
plurality of analog subsignals are received, synchronized, and
combined for reconstruction of the analog signal.
16. The claim of 14, wherein the synchronization signal is
transmitted without employing a coded chip.
17. A satellite communication system comprising: a transmitting
station receiving an original signal, dividing the original signal
into a plurality of subchannels to be transmitted over at least one
satellite transponder, and adding a synchronization signal and
propagation delay indicators to the plurality of subchannels
wherein the synchronization signal allows the plurality of
subchannels to be combined into a reconstructed original signal and
the propagation delay indicators allow the plurality of subchannels
to be properly combined into a reconstructed version of the
original signal; and a receiving station receiving the plurality of
subchannels from the at least one satellite transponder and
combining the subchannels according to the synchronization signal
and the propagation delay indicators to reconstruct the original
signal.
18. A satellite communication system comprising: a transmitting
station receiving an original signal, dividing the original signal
into a plurality of subchannels to be transmitted over at least one
satellite transponder, and adding a common timing signal to the
plurality of subchannels wherein the common timing signal allows
the plurality of subchannels to be combined into a reconstructed
original signal wherein each of the plurality of subchannels is
associated with a unique amount of propagation delay; and a
receiving station receiving the plurality of subchannels from the
at least one satellite transponder and combining the subchannels
according to the common timing signal to reconstruct the original
signal, the receiving station further comprising a subchannel
synchronization component that delays subchannels by a
predetermined amount based on order of arrival.
Description
BACKGROUND OF THE INVENTION
The invention relates to satellite communications systems
generally, and more particularly to satellite communication systems
which divide the transmitted signal, either in power or in content,
to be synchronized and recombined in the receiving terminal. This
concept applies readily to broadcast applications, but it not so
limited.
The satellite industry has experienced a progression of performance
enhancements evidenced by increased transmit power capability of
satellite transponders, improved low-noise amplifier (LNA)
characteristics, and smaller receiving antennas. In satellite
systems with a large number of receiving stations, it is
particularly important to reduce the cost of each receiving unit
and to design a system with a small receiving antenna to meet
installation and aesthetic requirements. The need for a small
receiving antenna has motivated an increase in transponder power
output in order to maintain an acceptable signal-to-noise ratio
(SNR) with the smaller antenna. As satellite users move from lower
power transponders to higher power transponders, falling demand for
the lower power transponders reduces the cost of their use.
Receiving a signal from a lower power transponder with the smaller
receiving antenna size produces a received power at the LNA too low
to maintain SNR requirements. The present invention permits the
receiver to combine received signals from a plurality of
transponders, possibly located on a plurality of satellites to
enable again the use of lower power transponders, but with small
receiving terminal antennas.
SUMMARY OF THE INVENTION
A satellite communications system includes a transmitting station
that directs information-carrying signals toward an orbiting
satellite, which receives the signals and in turn retransmits the
signals on a different frequency band toward a plurality of
receiving terminals. The satellite contains a transponder which
receives signals as a broad band of frequencies and retransmits
them on another set of frequencies of equal bandwidth but shifted
to another location in the spectrum.
The present invention has as its object a satellite communications
system including a transmitting facility that divides the signal
into a plurality of subchannels directed toward a plurality of
transponders located on one or more satellites and a receiving
terminal that receives the subchannels, time-synchronizes the
subchannels, and combines them into a faithful replica of the
original composite signal. The division of the signal into
subchannels is carried out by one of two methods. In a first
division method, the source signal is replicated across the
plurality of transponders. Hereinafter the first division method is
referred to as power-division. In a second division method, the
content of the source signal is represented by a set of distinct
subsignals, each of which subsignals contains less information as
the original signal, but said distinct subsignals can be
conveniently recombined in the receiver to reconstruct the original
signal satisfactorily. Hereinafter this second division method is
referred to as content-division.
In a system using power-division to create subchannels, the
originating transmitter directs more than one identical signal to a
plurality of transponders, possibly located on a plurality of
satellites. In said system, transponders retransmit and the
receiving antenna system conducts all of the aforementioned signals
into the receiving system. The receiving terminal provides means of
time-synchronizing the plurality of received signals, adjusts the
relative power level of the plurality of received signals to be
approximately equal, and combines the signals into a composite via
a signal adding process. Each of the signals added contains both an
information component and a random noise component, such noise
having been introduced primarily in the first amplifier of the
receiver, typically a low-noise block converter (LNB). Those
skilled in the art know that the information component of each
signal will be statistically correlated, but the noise components
will be statistically uncorrelated, both to each other and to the
information component. Thus the information components will add
linearly into the composite signal, that is in proportion to their
number. The power in the information component of the composite
signal will then be in proportion to the square of the number of
received signals being added together. In contrast, the power in
the noise component of the composite signal will be in proportion
to the number of received signals added together. Thus the SNR of
the composite signal is improved over the SNR of the individual
subchannel signals by a factor of N in power, where N is the number
of channels added together to form the composite signal. The
foregoing discussion assumes that the signal levels and noise
levels in each of the subchannel signals is identical.
In a real system, transmission characteristics will vary slightly
between subchannels, signal and noise levels being slightly
different between subchannels, resulting in an SNR improvement
ratio somewhat less than the factor of N described above. In any
case, the receiver may require automatic means of adjusting the
power of each of the signals to be added at the combining point so
as to be approximately equal to each other in level.
In a system using content-division to create subchannels, the
originating transmitter directs distinct subsignals toward the
plurality of transponders, the subsignals being created in such a
way as to permit convenient reconstruction of the original signal
at the receiving terminal. In an exemplary analog system, the
original signal can be divided into subband signals using a
filter-bank process. If the filters used satisfy quadrature-mirror
properties, the subsignals can be added directly to reproduce the
original signal without phase distortion at the boundary
frequencies. If the analog signal contains a strong periodic timing
component (as does a television signal), this periodic timing
component can be separated from the remainder of the signal before
dividing the signal into subband components. Said timing component
could then be added back to each of the subband components to
produce subchannel signals with different frequency components, but
common timing information. This strategy naturally provides timing
information useful to facilitate the necessary time-resynchronizing
process in the receiver.
As above, in a system using content-division to create subchannels,
the originating transmitter directs distinct subsignals toward the
plurality of transponders, the subsignals being created in such a
way as to permit convenient reconstruction of the original signal
at the receiving terminal. In an exemplary digital system, the
original binary signal can be divided into subchannel digital
signals, each of which has a bit rate less than the original
digital signal. The original digital signal can be divided into
subchannel digital signals in any number of ways. Two simple
exemplary digital subchannel strategies are as follows. A first
exemplary digital subchannel strategy is to direct each successive
bit into each subchannel sequentially. A second exemplary digital
subchannel strategy is to direct each fixed-size block of bits in
the original signal to each successive subchannel sequentially.
This second exemplary strategy fits well with digital source
signals that are organized in a fixed-block-size structure in the
original signal.
In the case that a plurality of satellites is used to conduct a set
of subchannels from a transmitting station to a given receiving
terminal, each subchannel will generally experience a different
propagation delay. The instant invention provides means to
determine the amount of time to delay each subchannel in order to
combine them synchronously. The delay required for each received
subchannel will in the general case differ. The present invention
provides additional means to implement the aforedetermined delay
for each subchannel independently.
The receiving terminal system, when activated for a particular
virtual channel, determines the relative delay between the
subchannel signals arriving at the receiver. This could be
accomplished by correlating the subchannel signals with each other
at all possible delays expected in a particular implementation of
the system. As this process is very time consuming and source
signal dependent, it is therefore subject to false synchronization
and possible failure to synchronize at all, particularly if the
source signal does not contain enough timing information. The
present invention solves this problem by transmitting a timing
signal along with the original signal. Said timing signal arrives
at the receiving terminal via a plurality of propagation paths,
each experiencing a different delay. The timing signal is added to
the virtual satellite system in such a way so as to be separable
from the original signal on each subchannel. The receiving terminal
then correlates timing signals arriving on different subchannels to
determine the amount of relative propagation delay. All subchannel
signals contain common timing information to facilitate the
correlation process. This guarantees that the subchannels can be
processed and compared in a known way to determine relative
propagation delay.
The timing signal can be added to the virtual satellite channel
using one of two exemplary methods, but the instant invention is
not so limited. A first exemplary method requires that a narrow
bandwidth signal be transmitted across each satellite in the
virtual channel. The narrow band signal requires a small allocation
of the available spectrum, but provides a dedicated timing signal
on each satellite actively carrying virtual satellite channels. The
narrow band timing signal provides propagation delay information to
virtual channel receiving terminals having one or more subchannels
on the satellite. The timing signal could consist of one or more of
the following exemplary signals, but the instant invention is not
so limited. A first exemplary signal is a carrier modulated
digitally by a binary pseudorandom noise sequence. A second
exemplary signal is a periodic pulse. The pulse could be
time-dispersed prior to transmission to create a signal with
improved peak to average waveform properties. The receiving
terminal in this example would reverse the time-dispersal process
to recover a narrow-time pulse. The time period of either exemplary
signal above described, after which the signal repeats, would be
longer than twice the greatest expected delay difference between
subchannels, thus facilitating unambiguous determination of
propagation delay.
A second exemplary method of incorporating a timing signal in the
virtual satellite system consists of adding a spread spectrum
component to each of the information-bearing subchannels in the
system, and within the bandwidth of each subchannel. The magnitude
of the spread spectrum timing component is much lower than the
information signal so as not to reduce the performance of the
normal receiver demodulation process. The spread spectrum signal is
then despread in the receiving terminal, thereby increasing its
magnitude above that of the information content. The increase in
signal level is proportional to the processing gain. This process
facilitates delay synchronization in the receiving terminal and has
two advantages. A first advantage is that the second exemplary
method does not increase the bandwidth requirements of the virtual
channel to accommodate a timing signal. A second advantage is that
the full bandwidth of the information channel is available to the
timing signal resulting in high resolution relative delay
estimation.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic and block diagram illustrating the present
invention.
FIG. 2 is a schematic and circuit block diagram of one embodiment
of the present invention.
FIG. 3 is a schematic and circuit block diagram of another
embodiment of the present invention.
FIG. 4 is a schematic and circuit block diagram of another
embodiment of the present invention.
DESCRIPTION OF THE INVENTION
Referring now to the drawings in which like reference numerals
indicate like or corresponding elements over the several views,
FIG. 1 shows an overview of the satellite communications system
consisting of subsystems 12, 10, 16. Original signal 22 feeds
subchannel divider 24 which separates the signal into a plurality
of numbered subsignals. The exemplary system of FIG. 1 shows the
number of subsignals to be four, but the present invention is not
so limited. Subchannel divider 24 creates the subsignals by
dividing original signal 22 employing one of two methods. A first
method divides the signal on the basis of power. In this first
method all the subchannel signals emerging from subchannel divider
24 are identical. A second method divides the signal on the basis
of content. In this second method, each subchannel signal carries
at least some information that is not carried by the other
subchannels. The information content may be mutually exclusive or
may overlap between subchannels, but in any case the subchannel
signals under the second divider method are not identical as in the
first method. Each subchannel signal feeds an uplink transmitter
26a 26d, each of which uplink transmitters feeds a separate antenna
28a 28d, directing radio frequency energy toward a plurality of
orbiting satellites 14a 14d via propagation paths 18a 18d. Uplink
transmitters 26a 26d add timing signal 23 to the signal to be
transmitted either on a separate frequency allocation or in the
bandwidth of the information-bearing carrier.
The exemplary system of FIG. 1 shows the number of satellites used
by the system to be four, but the instant invention is not so
limited. Each satellite 14a 14d receives a band of frequencies,
amplifies the signals received in that band, and retransmits the
band at a different location in the spectrum. Each of said
satellites has a transmitting antenna pattern that includes
receiving terminal system 16. Propagation paths 20a 20d from each
satellite 14a 14d to representative receiving terminal 16 carry
radio frequency energy from satellites 14a 14d to the receiving
terminal system 16. It should be understood that although FIG. 1
depicts each uplink signal being carried by a different satellite,
the present invention is not so limited. For example, transponders
of satellites 14a, 14b could be collocated on the same satellite.
In this case, uplink transmitters 26a, 26b and uplink antenna
systems 28a, 28b could be combined, in addition to satellites 14a,
14b representing the same satellite. Propagation paths 18a, 20a,
18b, 20b in this case would be combined into single uplink and
downlink propagation paths. Receiving terminal system 16
incorporates one of two antenna methods. A first method includes a
plurality of antenna components to receive the plurality of
satellite signals 20a 20d. A second method incorporates a multiple
beam antenna. The exemplary system of FIG. 1 uses multiple beam
antenna 30, but the present invention is not so limited. In either
of the aforementioned receiving terminal antenna methods, the
antenna subsystem produces a plurality of output signals
corresponding to the subchannel signals emerging from subchannel
divider 24 in uplink system 12. In the exemplary system of FIG. 1,
each of the numbered signals emerging from multiple beam antenna 30
correspond to similarly numbered signals emitted by subchannel
divider 24. This signal identity remains true whether satellites
14a, 14b of FIG. 1 are distinct or represent the same satellite as
indicated in the foregoing description. The subchannel signals
emitted by multiple beam antenna 30 feed a plurality of tuners 32
which then drive a plurality of demodulators 34. A signal emerging
from one of the demodulators 34 then represents a version of the
corresponding output of subchannel divider 24, but delayed in time
in proportion to the sum of the lengths of the corresponding uplink
and downlink propagation paths 18 and 20. In receiving terminal 16,
delay component 36 further delays first-arriving signals such that
all the subchannel components arrive at subchannel combiner 38 at
nearly the same time. Said combiner 38 produces a reconstruction 40
of original signal 22. The method used in subchannel combiner 38 is
consistent with and corresponds to the method used to divide
original signal 22 in subchannel divider 24.
(Digital Content-Division)
The instant invention uses one of three methods to affect the
dividing and combining operations of subchannel divider 24 and
subchannel combiner 40. In each of said methods, subchannel divider
24 of FIG. 1 feeds a plurality of uplink transmitters 26a 28b, but
the signals emerging from subchannel divider 24 are different in
nature depending of the dividing and combining method used. In a
first dividing and combining method, original signal 22 is digital.
In said first method, subchannel divider 24 divides said digital
signal into lower data rate subchannel signals with binary content
that contains at least some mutually exclusive information. The
division could be on a sequential bit-by-bit basis, could be on a
sequential frame-by-frame basis, and may or may not relate to
possible framing in the original digital signal. The exemplary
receiving terminal 16 of FIG. 2 depicts a two-subchannel digital
receiving system where the radio frequency carriers feeding the
demodulators 36a and 36b are quaternary phase shift keying (QPSK)
modulated signals, but the present invention is not so limited.
Said figure further indicates the use of a multiple beam antenna
30, but the present invention is not so limited. Referring again to
FIG. 2, multiple beam antenna 30 emits first and second signals
into first and second tuners 32a and 32b. Each tuner shifts a band
of higher frequencies to a band of lower frequencies of equal
bandwidth such that receiver controller 42 sets the center
frequency of the higher band, but the lower band is fixed. Tuners
32a, 32b emit QPSK modulated signals at a frequency that the QPSK
demodulators 36a, 36b expect to receive. As there are two
subchannels in the example of FIG. 2, the data rate of the binary
information contained in these QPSK signals is approximately half
the data rate of original signal 22. The respective outputs of QPSK
demodulators 36a, 36b emit signals to bit detectors 38a, 38b which
in turn produce streams of binary data corresponding to the
subchannel division in uplink system 12. Delay operators
synchronize the data streams by introducing delay in the
first-arriving binary stream such that there is a minimum of
relative delay between the respective delay operator outputs.
Digital content combiner 48 reverses the content division process
of subchannel divider 24 so as to produce at its output a faithful
delayed replica 50 of original digital signal 22. Receiver
controller 42 of FIG. 2 responds to user input (not depicted) to
select the transponders 14 to combine, subsequently emitting
control signals to multiple beam antenna 30 to direct its antenna
patterns toward the satellites containing selected transponders 14.
Receiver controller 42 also selects each tuner frequency consistent
with the signals emitted from the selected transponder. Receiver
controller 42 further processes information from timing signal
correlator 44 to determine the correct setting of delays 40a, 40b.
Timing signal correlator 44 receives and time-correlates tuner
outputs 34. For a system with more than two subchannels, correlator
44 processes tuner outputs in pairs to determine relative delay
between subchannels. Nonvolatile memory 46 contains parameters
regarding the user-selected transponders to enable the correct
setting of multiple beam antenna 30 and tuners 32.
(Digital Power-Division)
The instant invention can use a second method for transporting a
digital signal across a virtual satellite channel. Referring to
FIG. 3 which depicts an example of said second method which
combines delayed demodulator outputs from identical subchannels as
described previously as power combining. Under the direction of
receiver controller 42, multiple beam antenna 30 emits signals to
tuners 32a, 32b which translate variable transponder bands into a
fixed band of frequencies expected by the QPSK demodulators 54.
FIG. 3 depicts a receiving terminal using a multiple beam antenna,
but the present invention is not so limited. FIG. 3 further depicts
a receiving terminal with two subchannels, but the instant
invention is not limited to two subchannels. The figure in addition
shows the use of a QPSK modulation scheme, but the instant
invention is not so limited. Subchannel signals 52 emitted by
tuners 32 contain identical digital information transmitted at the
full rate of original signal 22. QPSK demodulators 54 produce soft
decision outputs I.sub.A and Q.sub.A for each subchannel. Since the
total propagation delay for each subchannel is in general
different, first-arriving soft decisions must be delayed in time by
an amount such that soft decisions emitted by delays 56 emerge with
nearly zero relative delay between subchannels. Delays 56 digitize
the analog soft decisions produced by demodulators 54, placing
digitized results in a first-in first-out (FIFO) buffer. Receiver
controller 42 controls the amount of time delay in delays 56 with
input from timing signal processor 44 and digital correlator 58.
Timing signal processor 44 analyzes input from tuner outputs 52 to
determine the relative time delay between subchannels. For systems
using more than two subchannels, the timing signal processor would
process subchannel tuner outputs in pairs. Since the subchannels of
FIG. 3 result from use of an uplink system 12 using power division,
delay outputs I.sub.B and Q.sub.B from delays 56a, 56b are
correlated. This enables digital correlator 58 to compare digitized
soft decisions between subchannels and provide additional
information to receiver controller 42 about relative subchannel
delay at the bit level. Digital power combiner 66 processes
synchronized I and Q soft decisions from all subchannels to produce
a single I and Q decision 68 for every set of soft decisions
presented. For the case of QPSK modulation, each final decision
from combiner 66 produces two bits in digital output 68.
(Analog Division)
A third method for dividing and combining the original signal
address the case that original signal 22 is analog in nature.
Referring to FIG. 4, receiver controller 42 directs multiple beam
antenna 30 to point to selected transponder signals and directs
tuners 32a, 32b to translate said transponder frequencies to a
fixed band of frequencies expected by demodulators 70a, 70b. The
exemplary system of FIG. 4 divides the signal into two subchannels,
but the instant invention is not so limited. Demodulators 70a, 70b
produce analog outputs signals which are faithful replicas of the
subchannel signals produced by subchannel divider 24 in the uplink
system 12. Said analog signal outputs in general experience
relative delay due to differing lengths of total propagation paths
when using transponders on different satellites. Under direction of
receiver controller 42, analog delays 72 add delay to
first-arriving subchannel signals so as to create outputs of analog
delays 72 which arrive at analog combiner 80 with near zero
relative delay. Analog delays 72 consist of a high quality
analog-to-digital converter (A/D), a FIFO buffer, and a
digital-to-analog (D/A) converter. Each delays 72 creates a time
delay in proportion the instant size of the FIFO buffer contained
therein. Delays 72 present output signals to analog combiner 80
which represent faithful replicas of the subchannel signals
produced by subchannel divider 24 in the uplink system 12. These
signals differ outputs of demodulators 70 in that they are now time
synchronized. FIG. 4 represents both signal division strategies,
power-division and content-division. In the first case of
power-divided subchannel signals, inputs to analog combiner 80
represent identical signals, differing only in distortion and noise
added by satellite transport. In a second case, time-synchronized
content-divided subchannel signals arrive at analog combiner 80.
Analog combiner 80 creates output 82 most likely by a simple
addition process, but is not so limited. In addition to producing
combined output signal 82, analog combiner 80 optionally provides a
measure of time synchronization to receiver controller 42 to
improve the accuracy of time alignment by controller 42. As in
first and second digital divider-combiner methods, timing signal
correlator 44 provides relative subchannel delay information to
receiver controller 42, which together with further optional delay
information from analog combiner 80 provides receiver controller 80
with a basis to create estimates of relative delay between
subchannels which in turn affects the setting of delays 72.
(Timing)
In first, second, and third divider-combiner methods, tuners 32
provide information to timing signal correlator 44 using one of two
timing methods. In a first timing method, receiver controller 42
adjusts tuners 32 to receive timing signal 23 placed on all
satellites with transponders used by the virtual satellite system.
In this first method, tuner adjustment is necessary as the timing
signals are placed at a frequency assignment separate form the
information-bearing transponder signal. This out-of-band timing
signal may be narrow-band in nature so as to conserve limited
bandwidth on the satellite and reduce system cost. In general,
timing signal 23 is unrelated to the information-bearing
transponder signal in either information content, modulation
strategy, or data rate or frame rate in the case of digital
transmission, but the present invention is not so limited. The
timing signal utilizes allocated bandwidth to enhance the
resolution of relative subchannel delay estimation. Possibilities
for the timing signals include pseudorandom noise, tone ranging,
and time-dispersed pulse, but the instant invention is not so
limited. A good timing signal must have a strong sharp
cross-correlation with a time-shifted version of itself and have
minimum spurious correlations. The instant invention includes two
timing signal processor methods. In a first timing processor
method, timing signal correlator 44 correlates output signals from
tuners 32 at various relative delays until an acceptable
correlation occurs indicating that the relative delay between the
subchannels has been reproduced in timing correlator 44. Receiver
controller 42 then sets analog delays 72 in accord with this
measured relative delay to synchronize inputs to analog combiner
80. In the case that there are more than two subchannels in the
virtual satellite channel, timing signal processor 44 compares
subchannel signals pairwise. In a second timing processor method,
timing signal correlator 44 correlates the output from each tuner
32 with a stored version of the known timing signal, or by
processing the recovered timing signal through a process that will
produce a periodic output in response to the timing signal. One
example of such a process is a matched filter, but the present
invention is not so limited. Once the delays 40, 56, 72 are
adjusted to remove relative subchannel delay, tuners 32 are set to
conduct the selected information-bearing transponder signals to the
respective demodulators in FIG. 1, FIG. 2, FIG. 3.
In a second timing method, the timing signal is as wide in
bandwidth as the information-bearing transponder and resides in
exactly the same bandwidth. In order to prevent distortion of the
information signal, the timing signal is greatly attenuated. In
order to recover the attenuated timing signal, timing signal
correlator 44 first processes the tuner outputs through a linear
system that creates a large processing gain to amplify the expected
timing signal above the output created by the presence of the
uncorrelated information-bearing carrier. The instant invention may
use one of three exemplary processes to recover a low-level in-band
timing signal, but the present invention is not so limited. In a
first exemplary process the timing signal is a time-dispersed pulse
with precise time dispersion introduced by a surface acoustic wave
(SAW) filter in timing signal generator 23. A matching SAW filter
in receiving terminal 16 performs the inverse of the dispersion
process, thus recovering the primary timing signal which is a
periodic narrow-time pulse. In a second exemplary process, the
timing signal is pseudorandom noise. Timing signal processor 44
then applies spread spectrum techniques to recover the timing of
the low-level in-band timing signal. Upon timing signal
acquisition, the correlated timing signal will experience a large
process gain, but the uncorrelated information carrier will remain
at the same relative level. This enables timing signal processor 44
to establish relative delay between subchannels, reporting results
to receiver controller 42. A third exemplary timing process uses a
multiple tone signal to establish timing. The sine waves selected
are harmonically related in such a way as to create a signal with a
relatively long period, but giving good time resolution with the
presence of some high frequencies. A linear filter at the selected
frequencies recovers the timing signal in favor of the information
carrier. Timing signal processor 44 then analyzes filtered timing
signals to establish relative time delay between subchannels.
In the case of the digital content-division receiver of FIG. 2,
there is typically no correlation between the subchannels to
provide feedback as to the accuracy of the delay settings of delays
40. This is a feedforward control system. Feedback is possible
however in the exemplary systems of FIG. 3, FIG. 4. Outputs from
delays 56 in the digital power-division receiver of FIG. 3 are
highly correlated. If the delay setting is slightly in error, a
local digital correlation reveals the necessary small correction.
Outputs from delays 72 in the analog receiver of FIG. 4 are
correlated to some extent depending on the nature of the analog
division and the instant properties of the analog content. This
provides optional feedback to receiver controller 42 to affect
local timing corrections.
While several particular forms and variations thereof have been
illustrated and described, it will be apparent that various
modifications can be made without departing from the spirit and
scope of the invention. Accordingly it is not intended that the
invention be limited, except by the appended claims.
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